This invention relates to a process for producing mixed poly(vinyl acetal)s and more particularly to a process for producing partial polyvinyl butyral containing unreacted vinyl alcohol groups which is modified with long chain acetal groups.
Polyvinyl butyral is used as sheet in transparent laminated glazings, adhesive compositions, protective coatings, photoimaging applications, binders for ceramics and the like. Such resin contains 10 to 30 weight % unreacted vinyl alcohol groups and is hereinafter called "partial polyvinyl butyral" or "partial PVB". Partial polyvinyl butyral can be modified by including long chain acetal groups in the polymer for internal plasticizing, such partial polyvinyl butyral containing long chain acetal groups hereinafter called "mixed poly(vinyl acetal)s". In this regard note U.S. Pat. No. 5,137,954, issued Aug. 11, 1992. Such long chain acetal groups can broaden the utility of a mixed poly(vinyl acetal). For example, U.S. Pat. No. 5,137,954 discloses use of mixed poly(vinyl acetal)s with reduced amounts of external plasticizer and U.S. Pat. No. 5,019,624 discloses mixed poly(vinyl acetal)s without external plasticizer.
The size of resin particles formed during synthesis of partial PVB and mixed poly(vinyl acetal)s is important, particularly for laminated glazing applications. Particles too large can vary in hydroxyl content within the particle and can appear as visual defects in the transparent sheet of the glazing; particles too small present handling problems during formulation with other materials and can be an explosion hazard.
Partial PVB and mixed poly(vinyl acetal)s have been synthesized by acetalizing polyvinyl alcohol with butyraldehyde or mixtures of butyraldehyde and another aldehyde at low temperature on the order of 5.degree.-20.degree. C., followed over time as the reaction continues by a slow increase to about 50.degree.-90.degree. C. In this regard, note Example 1 of U.S. Pat. No. 5,137,954. When two aldehydes were used in such prior art, simultaneous formation of both acetal linkages occurred during the acetalization reaction. As far as is known, this acetalization temperature profile in the prior art was apparently intended to provide the desired resin particle size and minimize problems just referred to.